The present invention relates to coated cemented carbide inserts with a binder phase enriched surface zone particularly useful for heavy roughing turning operations of very large steel components with improved edge security and wear resistance in combination with extremely good plastic deformation resistance.
For turning of steel materials, both stainless and normal steels, coated cemented carbide inserts are widely used, especially CVD-coated inserts with a binder phase enriched cemented carbide body.
Through the binder phase enriched surface zone, an extension of the application area is obtained. The far most commonly used type is a cemented carbide body with a from about 10 to about 30 μm thick surface zone that is essentially free from cubic phase and moderately enriched in binder phase. Examples are U.S. Pat. No. 4,277,283, U.S. Pat. No. 4,610,931, U.S. Pat. No. 4,830,283 and U.S. Pat. No. 5,106,674.
Another type of binder phase enriched cemented carbide inserts is the so called stratified type, which is accomplished using a powder with very carefully controlled carbon content and a sintering process with controlled cooling. This type has a surface zone from about 15 to about 45 μm thick, which is more strongly enriched in binder phase by several thin stratified layers of binder phase essentially parallel to the outer surface. In large scale production, the carbon control is difficult and the stratified binder phase enrichment is rarely used in practice.
EP-A-603143 discloses a cemented carbide with a binder phase enriched surface zone, said cemented carbide containing WC and cubic phases in a binder phase in which the binder phase enriched surface zone has an outer part essentially free of cubic phase and an inner part containing cubic phase and stratified binder phase layers. It is thus a combination of the abovementioned two types of binder phase enrichments.
Heavy duty machining operations to which the invention relates are characterized by the use of relatively high forces to shape workpieces both by non-cutting and cutting processes. Such operations include non-cutting shaping processes such as extruding, rolling, drawing and ironing and cutting processes such as punching, shearing, and broaching, as well as high force drilling, grinding, milling and turning processes. Characteristic of heavy duty machining operations, work done on the workpiece together with friction between the tool and workpiece generate sufficient heat to distort the workpiece and cause high rates of tool wear.
One example of a heavy duty application is heavy roughing turning operations of very large steel components such as drive shafts for ships and shafts for wind power plants which can be up to 20 m long and up to 1.5 m in diameter, where large cutting inserts are used with iC greater than or equal to about 19 mm, iC is the diameter of the inscribed circle of the insert, and a thickness of greater than or equal to about 6 mm. The demands on the cutting inserts are extremely high as the forged shafts often are oval and have forged skin and inclusions in the surface (oxide scale). With the ovality, the depth of cut (DOC) may be zero occasionally during the cutting operation. The steel type may be low alloyed or carbon steel, e.g., tough hardened steel. Furthermore, the cutting data is extreme with feed rates up to about 2.5 mm and depths of cut up to about 30 mm. This together puts very high demands on toughness behaviour, resistance to plastic deformation and wear resistance on the cutting tool insert.